It's difficult to answer your new question because it depends upon who's defining the meaning of the word 'should'. (A company shareholder might take a very different view to that of the employee's union representative). There's no statutory figure for terminating a contract by mutual agreement.
However, if the company was to make the employee redundant, he'd be entitled to a minimum of 12 weeks pay (capped at �310 per week) if the whole of his service occurred after the age of 41. To calculate the relevant amount for younger employees see here:
http://www.direct.gov.uk/en/Employment/Employe es/RedundancyAndLeavingYourJob/DG_10029836
The amount he would get if he was made redundant would seem to provide a starting point for any negotiations regarding a 'pay-off' for his voluntary departure.
Chris